Many people assume that medicines sold over the counter are inherently safe. After all, if you can buy something in a supermarket or high street pharmacy, how dangerous can it really be?

The reality is more complicated. Several commonly used over-the-counter medicines carry a real risk of dependence, misuse or harm when taken in higher than recommended doses, for longer than needed, or for the wrong reasons. Here are five medicines it is worth knowing about.

1. Codeine-based painkillers

Codeine is an opioid medicine used to treat mild to moderate pain and, in some formulations, to suppress coughing. Over the counter, it is usually combined with either ibuprofen or paracetamol. Once swallowed, the body converts codeine into morphine, which produces its pain-relieving effects.

Common side effects include drowsiness, constipation, nausea and dizziness. At higher doses, codeine can slow breathing and impair coordination. Some people are particularly vulnerable. Ultra-rapid metabolisers carry a genetic variant that causes them to convert codeine into morphine much faster than usual. This trait is more common in people of North African, Middle Eastern and Oceanian backgrounds and can lead to dangerous side effects even at standard doses.

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